The invention relates to apparatuses and methods for measuring the intracranial pressure (ICP), such as a mean ICP or absolute ICP or absolute mean intracranial pressure (ICP), of a mammalian subject, for example a person, noninvasively.
Standard methods for measuring intracranial pressure (ICP) involve drilling a hole through the skull and inserting an intraventricular catheter, subdural screw, or epidural sensor operatively connected to an external pressure sensor or transducer. These invasive methods cannot always be employed when desired or needed because they generally require a neurosurgeon to carry them out, are expensive to perform, and present risk of complications to the patient. It should also be plainly apparent as a matter of common sense that having to cut a patient's head open is a less desirable manner of obtaining an important parameter (intracranial pressure (ICP)) than obtaining the parameter without having to cut the patient's head open at all. There are limits to how often you can cut the patient's head open.
In order to overcome the problems of the invasive methods and to make ICP measuring more readily available considerable effort has been invested in devising non-invasive methods and devices of measuring ICP. U.S. Pat. Nos. 5,951,477 and 8,926,515 describe an ultrasonic Doppler device which detects the pulsatility indexes of the blood flow inside the eye artery for both intracranial and extracranial eye artery portions. The eye in which the blood flow is monitored is subjected to a small pressure, sufficient to equalize the pulsatility index measurements of the internal and external portions of the eye artery. The pressure at which such equalization occurs is used as a reference for autocalibration of the apparatus so that continuous absolute intracranial pressure measurements may be taken over a particular sampling period.
This method has a great number of drawbacks and limitations. One is that it does not take into account that pressure to the eye globe causes mechanical irritation of eyes. As a result an autonomous sympathetic apparatus of eye globs will cause reflector Cerebral Vascular Spasm (CVS), which in turn causes inaccuracy in the absolute ICP measurement, such that the results obtained do not exactly describe the real ICP. This is especially the case with the 80% of traumatic brain injury (TBI) patients that suffer from cerebral vasospasm (CV). Second, a great deal of inaccuracy is generated using this method because iatrogenic cerebral vascular spasm (CVS) after depression of the eye globe and closing of the superior and inferior ophthalmic and vortices retrobulbar veins, causes reduction of venous output from the eye globe to the intracranial space (Cavernous sinus).
Third, the results of transorbital dopplerography of ophthalmic arteries are variable and they depend on the operator and variability of anatomical features of each individual. Repeatability and reliability of dopplerography is very low.
Fourth, between 10% and 25% of patients with traumatic brain injury suffer from bilateral or unilateral periorbital ecchymosis, sub skin hemorrhages, sub skin facial-orbital emphysema, and orbital bone fractures and it is impossible to use the above mentioned method on these patients. For patients in intensive care units suffering from severe brain injury with elevated ICP from 20 to 60 mmHg, the accuracy of this method is low and impractical for wide use. Noninvasive ICP measurement as described in U.S. Pat. Nos. 5,951,477 and 8,926,515, to the extent that it can be used at all, can most probably be used only on healthy individuals.
Applicant is not aware of any widely used noninvasive method or apparatus for ICP monitoring or measurement. An article in Surgical Neurology International by Khan, Marium Naveed et al. published 5 Apr. 2017 entitled “Noninvasive monitoring intracranial pressure—A review of available modalities” reviewed 196 professional articles discussing 15 different noninvasive modalities and concluded that “there is still no noninvasive ICP monitoring modality available to replace the invasive techniques” after summarizing the situation as follows (citations omitted): “Intracranial pressure (ICP) is defined as the pressure inside the skull, and therefore, the pressure inside the brain tissue and the cerebrospinal fluid (CSF) . . . . . Normal ICP is usually considered to be 5-15 mmHg in a healthy supine adult, 3-7 mmHg in children, and 1.5-6 mmHg in infants. ICP >20 mmHg is considered to be elevated, and this is considered an important cause of secondary injury leading to irreversible brain injury and death. ICP monitoring is used in a number of conditions; traumatic brain injury, intracerebral hemorrhage, subarachnoid hemorrhage, hydrocephalus, malignant infarction, cerebral edema, CNS infections, hepatic encephalopathy, to name a few, and in all of these conditions ICP monitoring in the light of other parameters can influence management for better outcomes. There are several conditions where it is important to monitor ICP, as even minor fluctuations may require a change in management. The gold standard for monitoring ICP is an intraventricular catheter connected to an external pressure transducer; the catheter is placed into one of the ventricles through a burr hole. The catheter can also be used for therapeutic CSF drainage and for administration of drugs. Even though it remains an accurate and cost-effective method of ICP monitoring, it is associated with a number of complications. These include risk of infection, hemorrhage, obstruction, difficulty in placement, malposition, etc. Other invasive modalities for ICP monitoring, all of which entail the same complications as intraventricular catheter insertion, include intraparenchymal monitors, subdural, and epidural devices, as well as lumbar puncture measurements. Due to the number of complications associated with invasive ICP monitoring, researchers and clinicians have been trying to develop a reliable noninvasive modality for ICP monitoring. From the use of the Fontogram in the 1970s, to the ongoing experiments on acoustoelasticity effects on ICP, there is still no noninvasive ICP monitoring modality available to replace the invasive techniques.”